Permanent magnet excited electric motor

ABSTRACT

A permanent magnet excited electric motor is provided with a low circumferential hump-like protrusion on the face of each electromagnetic pole to partially decrease the air gap between the stator and rotor and act as a so-called magnetic cam to exert a force on the rotor to smooth an otherwise uneven parasitic slot torque that occurs between the interaction of slot openings of the electromagnetic poles and the gaps between the permanent magnet poles.

This application is a continuation of application Ser. No. 140,976 filedJan. 6, 1988, abandoned, which is a continuation of U.S. Ser. No. 002,957, filed Jan. 13, 1987 abandoned.

An electromagnetic current generator excited by permanent magnets,provided with a slotted laminated core

The present invention concerns an electromagnetic current generatorexcited by permanent magnets, which is provided with a slotted laminatedcore, such as a small brushless dc motor provided with permanent magnetrotor poles of relatively high inductance.

In so-called permanent magnet excited motors of this class (known inGermany by their acronym DEM), parasitic torque occurs on account of themutual interference between the slots and the edges of the magnetizedpoles, which prevents smooth running of the motor. If the openingbetween the magnetized poles is made narrower, the parasitic uneventorque component or so-called slot torque unevenness will be somewhatgreater.

If motors of this type are to be used in signal-processing devices whereextreme compactness is required, the rotor must be provided withso-called rare earth magnets, which are particularly powerful permanentmagnets. Such magnets may be made from a samarium and cobalt alloy. Whena motor of this type is used in a hard disk unit with disk diameters ofonly 31/2 or 51/4 in. and the motor must be installed inside the harddisk hub, whose outer diameter may be a mere 40 mm or even 25 mm, ifrelatively high performance is to be obtained from so restricted a spacepackage, apart from a rare earth magnet to provide high air gapinductance, a multiphase coil must be used as well, although extrememiniaturization makes such devices difficult to manufacture. In a designwith an external rotor, the essentially cylindrical air gap ofrelatively reduced size will generally be arranged radially outside theouter cylindrical surface of the stator.

The permanent magnet rotor is generally made of thin half or quartershells, or rings which are narrow in the radial direction and areaxially superimposed on one another. The radial narrowness of such rotormagnets results from the extremely limited diameter of the motor. Inextremely small high output motors the air gap must be made as narrow aspossible as well, whereby the term air gap is understood as theelectrically effective mean distance between the iron surfaces of thestator and rotor. The combination of high inductance, a slotted statorand a minimum air gap thus gives rise to the extremely acute problem ofreducing, or if possible, altogether preventing the occurrence ofso-called parasitic slot torque unevenness in a motor intended toprovide as constant torque as possible.

Furthermore, although trapezoidal magnetization of the permanent magnetrotor is associated with certain performance advantages, this methodwill accentuate the above problem, compared with sinusoidalmagnetization.

A means of reducing the third harmonic of the torque pattern is taughtin document DE-OS 28 23 208, whereby a certain periodicity is impartedto the stator air gap surface, so that the air gap varies periodicallyover the circumference of the stator.

The purpose of the present invention is to provide smooth running inmotors of the aforesaid type, despite the presence of high fielddensities at the air gap. This purpose is implemented by the meansdescribed in claim 1.

The low peripheral hump-like protrusions according to the presentinvention serve to partially narrow the air gap above the stator poleand act, in a manner of speaking, as magnetic cams, whereby they exertforces on the rotor which can be used to smooth the torque. Thegenerally somewhat rectangular cross section of these cams approachesthe center, or even extends as far as the center of the stator poles inone significant embodiment of the present invention.

The present invention is preferably employed in motors of the aforesaidtype provided with a cylindrical air gap and a slotted inner stator witha multiphase unoverlapped winding.

Further developments of the present invention and/or some advantageousembodiments thereof, will be derived from the subclaims.

The drawings are described as follows:

FIG. 1a shows a developed longitudinal section of the first embodimentof a motor according to the present invention, whereby the ratio of thenumber of stator poles to rotor poles may be 3:2 or 6:4, for instance.

FIG. 1b shows the corresponding patterns of parasitic slot torqueunevenness 1 and useful torque unevenness 2 according to the presentinvention.

FIG. 1c shows combined curves 1 and 2.

FIG. 1d is an enlargement of a portion of FIG. 1a.

FIGS. 2a, 2b show an alternative and/or additional element whose effectis similar to that of the first embodiment according to FIG. 1a.

FIGS. 3a, 3b show an alternative solution to that shown in FIGS. 2a and2b, which is easier to manufacture, whereby FIGS. 2a, 3a show theleading face in an axial direction and FIGS. 2b, 3b show a cross sectionof the aspect according to FIGS. 2a and/or 3a.

FIG. 4 shows essentially a view of the first embodiment according toFIG. 1a, enlarged approximately 4 times life-size, whereby the air gapagain narrows towards the stator pole end until it corresponds to thecylindrical enveloping surface, which also surrounds the raisedprotrusions according to the present invention.

FIG. 5 shows a second embodiment provided with pointed pole ends in aradial direction.

FIGS. 6a, 6b show an enlarged view of the stator configuration accordingto FIGS. 4 and 5.

These drawings will now be described individually. FIG. 1 shows thepartial development of a 6-pole stator provided with concentrated poles,11, 12, 13 and a 4-pole rotor provided with permanent magnet poles 21,22, 23, between which narrow circumferential pole gaps 25, 26 arearranged, wherein the width `s` of the stator slot openings 14, 15corresponds to the distance between two stator pole ends, whereinfurthermore the magnetic cams 3 (items 113, 123, 133), the extent ofwhose projection is labeled `a`, are arranged concentrically at thecenter of each stator pole, wherein the height of these cams is labeled`h`. Between the stator 10 and the rotor 20, the air gap is found, whichis defined by the stator pole heads and the permanent magnets 21, 22 and23 of the rotor. The four rotor poles 21 et seq are surrounded by a softmagnetic return circuit layer 24. During motor rotation, i.e. when therotor moves in the direction of the arrow 27, uneven parasitic slottorque arises as a result of the interaction between the stator slotopening 14 and the rotor pole gap 25, resulting in the uneven torquecurve 1 shown in FIG. 1b. At the same time, over a distancecorresponding to the rotor/pole pitch T_(p) rotor, as result of theinteraction between the magnetic cam 133 and the rotor pole gap 26,uneven counterphased torque occurs adjacent to the rotor pole gap 26according to the curve 2. FIG. 1c shows the compounded curves 1 and 2.Parasitic slot torque arises whenever the end of a high inductancepermanent magnet, such as the magnet 21, adjacent to the pole gap 25,travels from the stator pole 11 past the open slot 14 above the adjacentstator pole 12. This has been confirmed by measurement which has shownthat whenever the edge of the slot 14, for instance, is located betweenpoles, such as poles 21,22 for example, maximum torque unevennessarises, as can be seen from the curve pattern 1 according to FIG. 1b.Consequently, the distance between the peaks of curve 1 corresponds tothe width `s` of the slot openings, such as 14, for instance.

In order to make the distance b between the compensation torque peaks(curve 2) provided by the cams 3 according to the present inventioncoincide with this distance `s`, it is important to optimize the camwidth a to this end. This is because at b=s the parasitic torque curve 1will be fully compensated by the curve 2. This optimal cam width lies inthe range n=0.5 . . . 1xs but it will be influenced by the inductancepattern in the permanent magnet.

It has thus been demonstrated that uneven torque can also be generatedby the presence of narrow air gap constrictions above a stator pole inthe circumferential direction, whose pattern can be made to almostcoincide with that of the slot openings (cf. curve 1), by suitablepositioning and sizing of the so-called magnetic cams 3. If these camsare arranged so that the average distance between them and thecorresponding slot gap equals one pole pitch of the rotor (p) or awhole-number multiple thereof, the uneven torque pattern provided bythese cams will be the exact reverse of the slot torque, i.e. slottorque unevenness will be compensated (cf. curve 2).

The circumferential width a of this cam, that is to say its extent in`a` circumferential direction, may not amount to more than a minutefraction of the stator pole pitch. An optimal relationship existsbetween the circumferential width of the cam `a` and the width `s` ofthe slot gap between the aforesaid stator poles or salients, so thateven an extremely narrow cam width 8, accompanied, under certaincircumstances, by simultaneous widening of the slot, may produce thesame ratios, which is advantageous for the coil winding process.

In particular, in the case of a 3-phase winding with 4 rotor poles for asix-slot motor of the type described in disclosure document DE-OS 31 22049, the invention has been shown to be capable of achieving anoutstanding reduction of undesirable slot torque unevenness.

On the basis of the results of optimization testing, it was observedthat a further improvement could be obtained by narrowing the air gaptowards the ends of the stator hammer heads by the provision of radialprotrusions, in the form of pointed tips at the ends of the statorpoles, as described in disclosure documents DE-OS 29 19 581 or 30 49 494in connection with so-called reluctance torque motors.

Providing the hammer heads with pointed ends (6) according to FIG. 5 hasa similar effect to the presence of the cams. However, because thedistance `A` will always exceed the width of the slot opening `s`, theamplitude of its curve 2^(x) will always be less and its position willbe displaced away from the center of the slot. As a result, a relativelylarge harmonic will be produced, and consequently in an optimalarrangement these pointed tips will always be combined with an evennarrower cam, where a^(x) =0.3×5 to 0.8×5. It was surprising to findthat the same ratios prevail if the cam height is reduced to h^(x)=0.05-0.2 mm, which allows a further favorable reduction of the averageair gap width.

This is shown in FIGS. 1a, 1b, whereby the second solution correspondingto claim 6 is shown by lines or dashes and dots and dotted lines. Thecompensatory effect of the superimposed pointed tips 111, 112corresponds to the curve 2^(x), and the effect of the peripherallynarrower cam 3^(x) corresponds to the curve 2^(xx) (cf. curve 1b). Bothof these effects occur as part of the overall effect at the same time asthe curve 2, but this result is obtained at a lower value of h^(x), sothat the average air gap is narrower and increased motor output isobtained. This surprising development is all the more welcome, since acertain allowance for manufacturing tolerances must be made in motorconstruction, in addition to the theoretical minimum air gap, and insizing the air gaps in motors of this small size every tenth of amillimeter is significant.

The cam height `h` defines the peak amplitude. This is implanted withinthe usual range of air gap widths of 0.3-0.6 mm, as h=0.1-0.3 mm highrange in the design of a motor according to the present invention (seeFIG. 4). In order to make the average air gap expansion resulting fromthe presence of the cams as small as possible, it is furthermoreproposed that the diameter in the direction of the slot opening beincreased progressively towards the cam height.

In an alternative or possibly additional arrangement, a cam effect canbe obtained in the slot opening area by magnetically sealing the area bymeans of ferromagnetic slot parts, as taught in document DE-AS 11 94043.

It would basically appear possible to use the present invention inconnection with slotted rotor cores, as well. If these rotor coresrotate in the magnetic field of the permanent stator magnets, enhancedsmooth running can be obtained in this area as well, according to thepresent invention. This variant however implies the need to providemechanical brushes. Reduced slot torque unevenness can be useful inmotors of this type to provide enhanced running characteristics forspecial applications.

FIG. 2a shows ferromagnetic brackets 33 riveted to the stator cores,whose branches 34 in an axial direction project beyond the core package36, so as to narrow the air gap between the cores and the outer rotormagnet N. For this reason, these axial branches 34 of the brackets 33have the same effect at the pole center as the magnetic cams accordingto the present invention. This solution is certainly a compromise whoseoverall significance lies in that it provides a relatively wideeffective air gap between the rotor and the stator (N, 36). However, nospecial stamping tool is required for the manufacture of the corelaminations 36. The aforesaid brackets are provided in an axialdirection on both sides of the face of the stator core package, as shownin FIG. 2b. In combination with the brackets 33 which project over thestator laminations in an axial direction, and in particular the axialbranches of these brackets, the rotor magnet which projects in an axialdirection provides an effective and low cost application of the presentinvention and a possible enhancement of the effectiveness of thearrangement according to FIG. 1.

FIGS. 3a, 3b show a way of substituting the brackets 34 by angled tabinserts 37 provided with projecting axial collars.

In FIG. 5 a narrowing of the air gap towards the stator pole endsindicated by lines of dots and dashes, is provided so that thecorresponding torque peaks (curves 2^(x)) are further apart than thewidth of the slot opening `s`, corresponding to the distance A. Thiseffect was described in connection with FIG. 1. Adjacent the area aboutthe center point between the stator pole midpoint and any stator poleend, a circular cylindrical protrusion 102, 105 is provided beyond thecylindrical envelope surface 103, starting at the recess near the cam 3xand extending to the recess near the tip 111 of the stator pole end, sothat the average air gap diameter is effectively reduced (cf. FIG. 6b inconnection with the stator pole ST II). The radius of curvature `R` ofthis protrusion 102, 105 is substantially less than that of the envelopesurface 103, 100, 104.

I claim:
 1. An electromagnetic direct current motor excited by permanentmagnets comprisinga laminated core having radially extending slots withslot openings at a coaxial surface of the core, the coaxial surfacebeing cylindrical; at least one winding forming electromagnetic polesdefined in sectors between the slots, a plurality of radially thin,annular permanent magnets arranged in alternating polarities in thegeneral form of a ring with a gap separating each polarity, the faces ofthe electromagnetic poles facing the faces of the permanent magnets andbeing separated by an air gap defined therebetween, one of theelectromagnetic poles as an entity or the permanent magnets as an entitybeing the stator and the other the rotor; no more than one raisedportion on each of the electromagnetic pole surfaces extending radiallyinto the air gap, the circumferential width of the raised portion beingsmall compared to the electromagnetic pole pitch such that theelectromagnetic pole pitch is many times the circumferential width ofthe raised portion; and the raised portions being locatedcircumferentially on the surface of the laminated core such that thedistance measured in the direction of rotor rotation from the center ofa core slot defined between any given pair of electromagnetic poles tothe center of a raised portion on the electromagnetic pole adjacent thegiven pair is equal to n times the permanent magnet pole pitch, n beinga whole number.
 2. An electromagnetic direct current motor according toclaim 1, wherein the motor is a small brushless dc motor and wherein thestator includes the laminated core and electromagnetic poles and therotor includes the permanent magnet poles, the rotor poles being ofrelatively high inductance.
 3. A motor according to claim 2, wherein theraised portions are provided at the stator pole centers, and wherein theratio of the number of stator poles to rotor poles is 3:2.
 4. A motoraccording to claim 3, wherein the circumferential extent of the raisedportion corresponds to from 0.5 to 1 times the width of the stator slotopening.
 5. A motor according to any one of claims 2-4, wherein the airgap is maximally wide immediately adjacent the raised portions, thewidth of the air gap decreasing to the value above the raised portion asthe air gap approaches the stator pole tips.
 6. A motor according toclaim 5, wherein at an effective air gap of from 0.3 to 0.6 mm, theheight of the raised portion is from 0.1 to 0.3 mm.
 7. A motor accordingto claim 5, wherein the electromagnetic pole faces are pointed in aradial direction towards the air gap and wherein the circumferentialextent of the raised portion corresponds to from 0.3 to 0.8 times thewidth of the stator slot opening.
 8. A motor according to claim 7,wherein the height of the raised portion is approximately from 0.05 to0.15 mm at an effective air gap width of from 0.3 to 0.6 mm.
 9. A motoraccording to claim 8, wherein the width of the air gap decreasesprogressively from immediately adjacent the raised portioncircumferentially toward an adjacent slot opening.
 10. A motor accordingto claim 8, wherein the width of the air gap decreases in steps fromimmediately adjacent the raised portion circumferentially toward anadjacent slot opening.
 11. A motor according to claim 3, wherein thestator winding provided with a cylindrical air gap is multiphased.
 12. Amotor according to claim 11, wherein the stator winding is wound withoutoverlap.
 13. A motor according to claim 2, wherein the pattern ofmagnetization of the rotor in the circumferential direction isdistributed in a trapezoidal shape.
 14. A motor according to claim 13,wherein the rotor magnet is a rare earth magnet.
 15. A motor accordingto claim 12, wherein the stator is a three-phase stator.
 16. A motoraccording to claim 15, wherein the motor is provided with 6 stator polesand 4 rotor poles.
 17. A motor according to claim 9, wherein adjacentthe raised portion, the air gap width immediately decreases sharply,then increases and finally decreases again as it approaches the statorpole tips.
 18. A motor according to claim 6 having an outside diameterof approximately 40 mm.
 19. A motor according to claim 6 having anoutside diameter of approximately 25 mm.
 20. A motor according to claim10 wherein adjacent the raised portion, the air gap immediatelydecreases sharply, then increases and finally decreases again as itapproaches the stator pole tips.
 21. A motor according to claim 8 havingan outside diameter of approximately 40 mm.
 22. A motor according toclaim 8 having an outside diameter of approximately 25 mm.